The 26-year-old South Korean recovered from a bogey on No. 17 with a birdie on the final hole to finish at 11-under 277 and win for the fifth time on the LPGA Tour.
The coronavirus pandemic forced the tour to cancel dozens of its events including the Evian Championship this year.
Kim birdied her final three holes and tied the record for the largest comeback in a US Women's Open.
The 20-year-old Japanese rank outsider, who earned her nickname because of her radiant smile, rammed home a lengthy birdie putt of just under 20 feet at the final hole to edge out American Lizette Salas by one stroke.
Despite two bogeys in the final three holes, a one-under-par 70 was enough to clinch the title on a day when most of the contenders struggled at Country Club of Charleston.
The 23-year-old South Korean won last month's LPGA Founders Cup, captured her first major crown earlier this month at the ANA Inspiration and could claim her third title in five starts this week at par-71 Wilshire Country Club.
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